1. Field of the Invention
This invention relates to an adhesive for metals and more particularly to an adhesive for metal containers in which the end and body members are joined through a lap seam.
2. The Prior Art
There has been developed metal containers formed from aluminum and steel of the two-piece type which are formed from a dome-shaped end member having a depending annular lip or skirt and a body member, the body member being a seamless cylinder having an integral bottom, the open end of the body member being formed to be inserted in telescoping relationship with the annular lip of the end, the overlapping sections of annular lip of the end and the open body section being bonded together to form a leakproof joint by the interposition of a suitable adhesive between the opposed faces of the end and open body sections.
In copending application Ser. No. 191,226, filed Sept. 26, 1980, in the name of Donald J. Roth, the disclosure of which is incorporated herein by reference, there is disclosed a two-piece container of this type suitable for use in the packaging of carbonated beverages such as beer and soft drinks wherein the end member is of a relatively short length compared with the body member, the end member being comprised of a conical section with a central pour opening and a toroidal peripheral section about the conical section and annular lip or skirt depending therefrom. The upper, open end of the body member is necked in to form an axially extending cylindrical portion or annulus of a diameter reduction to complementarily fit into the lip of the end, the outer surface of the annulus being adhesively bonded to the inner surface of the end lip to form a lap joint between the annulus and lip sections of the body and end members.
The end and body members have wall thicknesses less than 10 mils and are pliable without permanent deformity to accommodate laterally imposed loads. When the assembled container is filled with a carbonated beverage, the adhesively bonded end reacts to the internal forces generated by the pressurized beverage to expand into a spherical shape whereby the lip is caused to be beamed radially inwardly against the annulus section of the body. The forces tending to expand the end member are thereby utilized to produce a compressive force radially inwardly on the lip together with the tensile forces exerted by the pressurized beverage tending to expand the upper end of the body insures a parallelism between body and lip portions precluding the development of voids which would produce leaking joints. The force couple created between the lip and the necked in body portions causes the adhesive interposed therebetween to be held in compression whereby peeling forces on the bonded lap joint are minimized or substantially eliminated.
The construction of the two-piece containers disclosed in Ser. No. 191,226 lends itself to high speed production in which an adhesive composition dispersed in a volatile solvent is applied, using conventional coating techniques such as brush, roll coating or gravure coating, is either applied to the outer surface of the annulus of the necked in body section or to the inner surface of the end lip prior to assembly of the end and body members or both the annulus and the lip. After application of the adhesive, the coated member is heated to volatilize the solvent media in which the adhesive is dispersed and the end member is guided into an interference fit assembly with the annulus of the necked-in body section. After assembly, the container is heated to an elevated temperature to the softening temperature of the adhesive to cause the adhesive to bond the assembled end and body members.
The high strength adhesives known to the art have not been totally successful in bonding the end and body portions of the two-piece containers discussed above for a number of reasons. Some of the major reasons are that although these adhesives have excellent cohesive strength, they are deficient either in their adhesion to the outer metal surface of the annulus or to the coated interior surface of the end lip.
The body member of the two-piece container of Ser. No. 191,226 is readily fabricated by the well-known drawn and wall ironing process. The body members fabricated by this process have outer wall surfaces which are highly polished and extremely smooth which makes it difficult to effectively bond thermoplastic adhesives such as polyvinylchloride to such surfaces. Although vinylchloride polymer adhesives are extremely compatible and will strongly bond to the vinylchloride polymer coatings conventionally used as interior coatings for beverage containers, such adhesives will not adhere effectively to the polished exterior surface of the drawn and wall ironed container body. As the construction of the two-piece container of Ser. No. 191,226 requires that the outer surface of the annulus of the necked-in section of the body be overlapped and bonded to the interior coated surface of the end, thermoplastic adhesives such as polyvinylchloride, polyethylene, polypropylene do not form adhesive bonds with the reliability required in the manufacture of containers used in the packaging of carbonated beverages.
Attempts at using thermosetting resin compositions such as those based on epoxy resins have revealed that although such compositions will strongly adhere to polished metal surfaces of the character found on the outer surfaces of drawn and wall ironed container bodies, such adhesives do not have sufficient adhesion to coated metal surfaces of the type found in the interior surfaces of containers used for packaging carbonated beverages, e.g., vinyl chloride polymer coated metal substrates, to provide an adhesive bond of the reliability required in the manufacture of containers used for the packaging of carbonated beverages.
It would be desirable if the excellent adhesion of thermoplastic resins such as vinyl chloride polymers to internal coated metal surfaces could be combined with the polished surface adhesion of thermosetting resins such as epoxy resins to obtain an adhesive formulation which would adhere tenaciously both to polished metal surfaces as well as coated metal substrates.
Attempts to disperse a combination of thermosetting resins such as epoxy resins and thermoplastic resins such as vinyl chloride polymers in a common dispersion media have not been successful as the two resin types are incompatible with the result that unstable dispersions result which rapidly stratify on standing, thereby rendering the dispersions unsuitable for commercial practice.